I. The structure of the primary ending of the living mammalian muscle spindle would be studied using fluorescent dyes as well as Nomarksi images. Dimensional changes of the terminals will be examined at different spindle lengths. A 3-dimensional reconstruction would be made permitting a quantitative description of the geometry of the ending. A model of the spindle derived from these data will be useful in understanding the function of this important mechanoreceptor. Changes in geometry with suggest possible modes of transduction. II. Relation between motor axon conduction velocity and muscle fiber innervation. Deoxyglucose will be used to label muscle fibers in one of the 2 heads of superficial lumbrical muscle of cat activated by stimulation of a single motor axon. Sections will be used for autoradiography and for histochemistry. The numbers and cross sectional areas of the activated fibers will be related to conduction velocity of the motor axon to determine which factors cause the precise relation between motor unit force and conduction velocity in fatigue resistant units.